1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly, to a stepper having a exposure time monitor for identifying a degraded lamp condition.
2. Description of the Related Art
Semiconductor devices, or microchips, are manufactured from wafers of a substrate material. Layers of materials are added, removed, and/or treated during fabrication to create the electrical circuits that make up the device. The fabrication essentially comprises four basic operations. The four operations are:
layering, or adding thin layers of various materials to a wafer from which a semiconductor is produced; PA1 patterning, or removing selected portions of added layers; PA1 doping, or placing specific amounts of dopants in the wafer surface through openings in the added layers; and PA1 heat treatment, or heating and cooling the materials to produce desired effects in the processed wafer.
Although there are only four basic operations, they can be combined in hundreds of different ways, depending upon the particular fabrication process. Indeed, whole texts have been written on the many ways in which these operations can be employed. See, e.g., Peter Van Zant, Microchip Fabrication A Practical Guide to Semiconductor Processing (3d Ed. 1997 McGraw-Hill Companies, Inc.) (ISBN 0-07-067250-4).
Patterning is also sometimes called photolithography, photomasking, masking, oxide removal, metal removal, and microlithography. The term "photolithography" will hereafter be used to refer to patterning operations. In photolithography, typically, a machine called a "stepper" positions a portion of a wafer being processed under a "reticle," or photomask. A reticle is a pattern created in a layer of chrome on a glass plate. Light is then shone onto the wafer through the reticle. The chrome blocks some of the light. The light shining through the pattern on the reticle changes the material characteristics of the wafer where it contacts the wafer. These changes make the material more or less susceptible to removal in another operation, depending on the particular process being implemented. The stepper then positions another portion of the wafer under the reticle, and the operation is repeated. This process is repeated until the entire wafer has undergone the operation.
Next, portions of the photoresist layer are removed to expose selected portions of the underlying process layer. Thereafter, typically through one or more etching processes, the exposed portions of the underlying process layer are removed to define a pattern in the underlying process layer. The purpose of photolithography is to define what will ultimately become patterns in or on a layer on a wafer, the parts of which may ultimately become parts of the semiconductor device. These patterns in the layer of photoresist must be laid down precisely in the exact dimensions, within certain manufacturing tolerances, required by the circuit design and to locate them in their proper place.
The photolithography operations generally set the "critical dimensions" of the semiconductor devices (e.g., the width of the gate conductor in an illustrative field effect transistor). Errors in the photolithography process can cause a whole host of problems including, but not limited to, distorted patterns, misplaced patterns, and other defects. These types of errors can ultimately result in undesirable changes in the functioning of the electrical circuits so that the wafer has to be scrapped. Photolithography processes are performed at very small dimensions, so that they are also highly susceptible to contamination by unwanted variations in processing conditions.
The light source, also referred to as a lamp, used in a stepper to expose a wafer degrades over time due to repeated uses. Typical control circuitry on any number of commercially available steppers automatically adjusts the exposure time of the lamp to account for the degradation. The stepper control circuitry monitors the exposure dose of light received by the wafer and maintains a constant exposure dose by increasing the exposure time accordingly. Thus, lamp degradation does not result in variations in the exposure dose received by the photoresist layer being patterned and negatively impact the photolithography process.
Certain classes of photoresist, sometimes referred to as chemically amplified resists, employ a post exposure bake process after exposure in the stepper to bring the patterning step to fruition. During the post exposure bake, hydrogen free radicals, formed in small quantities on the surface of the photoresist layer during the exposure in the stepper, diffuse through the photoresist layer and foster the chemical reactions necessary to change the solubility of the exposed portions such that either the exposed portions or the unexposed portions may be removed in a subsequent developing step.
There comes a point in the stepper exposure time automatic control process where the exposure time is increased to such a level as to cause heating affects in the photoresist layer being exposed. These heating effects may cause undesirable diffusion of the hydrogen free radicals before desired, thus altering the pattern formed in the photoresist and possibly resulting in the scrapping of the wafer being processed. A common technique for preventing the increasing exposure time from deleteriously affecting the processing of wafers involves periodically changing the lamp on a fixed interval (e.g., after a predetermined number of processing runs). Because lamps do not always degrade at the same rate, the replacement interval is typically selected using a conservative number of runs. If the replacement interval is overly conservative, equipment down time and maintenance costs negatively impact the efficiency of the stepper. If the replacement interval is too great, heating affects can damage wafers being processed, also decreasing the efficiency of the stepper.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.